As the next generation of networking occurs, organizations are becoming increasingly reliant on their networks to deliver Internet Protocol (IP) communications and mission-critical information. With the trends towards IP telephony and converged applications becoming a reality, there is now a greater need to incorporate QoS into the network infrastructure. QoS comprises a set of mechanisms that gives priority to delay-sensitive applications and makes the network more efficient and reliable for all applications.
QoS is designed to prioritize traffic and allocate network resources so that information arrives smoothly and predictably at its destination. It enables traffic to be grouped into categories based on common characteristics, allowing prioritization and services to be applied at the user or application level. Priority levels range from “mission-critical” (highest priority) to “best effort” (lowest priority). While over-provisioning bandwidth is an alternative to using QoS, and is an effective way to manage bandwidth in some networks, it cannot provide any guarantees that delay-sensitive traffic, such as voice and video, will arrive at its destination as the sender intends. QoS can make more efficient use of bandwidth and traffic management without adding capacity, and is therefore an attractive way to meet the needs of delay-sensitive traffic and to make better use of enterprise resources (e.g., bandwidth and equipment investment).
QoS depends upon the per-hop behavior (PHB) of the packets that constitute the traffic the network conveys. In turn, the PHB depends upon the transit time of the packets as they cross various network nodes in their journeys across the network. Unfortunately, nodes can become overutilized and congested, making the PHB more difficult to predict and manage, and degrading QoS.